Make setTestMode() a module API

Publish setTestMode() as a module API so that mainline modules can use
it in their test code.  The API is flagged.

Also, test APIs are now guarded so that they throw if the current
process is not a test.  This behavior is gated by an existing PIC test
flag.

Flag: android.os.ipc_data_cache_test_apis
Flag: android.app.enforce_pic_testmode_protocol
Bug: 396173886
Test: atest
 * FrameworksCoreTests:PropertyInvalidatedCacheTests
 * FrameworksCoreTests:IpcDataCacheTest
 * CtsOsTestCases:IpcDataCacheTest
Change-Id: I1cf9bacb676c12d5821959e4cc672df6339a2d42