perfetto: Add high-performance Java data source and ProtoWriter

Add a zero-allocation Java data source API for writing custom trace
data from performance-critical code paths (e.g., frame rendering).

The core of this is ProtoWriter, a pure Java protobuf encoder that
writes directly to a pre-allocated thread-local byte buffer using
the same 4-byte redundant varint trick as C protozero for nested
message length fields. This enables single-pass encoding without
size pre-computation.

Architecture:
- ProtoWriter: zero-alloc proto encoder (varint, fixed, string,
  nested messages). ASCII fast path for strings. UTF-8 fallback
  with pre-allocated scratch buffer.
- PerfettoDataSource: base class with volatile enabled flag (~1ns
  disabled check) and ThreadLocal TraceContext.
- TraceContext: per-thread writer + optional InternPool. Single
  JNI call (nativeWritePacketToAllInstances) writes encoded bytes
  to all active tracing session instances via the C SDK stream
  writer, which handles chunk boundaries and patching.
- InternPool: optional string interning with lazy incremental
  state checking (no JNI overhead when interning is unused).

The JNI layer correctly handles:
- Multi-instance: iterates all active sessions, writes to each.
- Enabled flag: uses native enabled_ptr to avoid disabling when
  other instances remain active.
- Incremental state: tracks per-instance clear flags for interning.
- Large packets: delegates to PerfettoStreamWriterAppendBytes
  which handles cross-chunk writes and size field patching.

Benchmarks (Java program, real JNI, real Perfetto shmem, Linux):
  Full E2E encode + JNI + shmem (basic):  ~112 ns/op
  Full E2E encode + JNI + shmem (+ arg):  ~123 ns/op
  ProtoWriter encode only:                 ~14 ns/op
  JNI + shmem only (pre-encoded):          ~95 ns/op
vs C SDK raw data source (same-size pkt):  ~77 ns/op (1.45x)
vs C SDK track event LL:                  ~192 ns/op
vs existing Java track event HL:          ~179 ns/op

Bug: N/A
Test: perfetto_integrationtests --gtest_filter="JavaDataSourceTest.*"
Test: ProtoWriterTest, InternPoolTest, FullTraceE2ETest (JUnit4)
Change-Id: I0000000000000000000000000000000000000000