libbinder_ndk: compiles w/o weak symbols

Android platform builds with weak symbols, but apps don't.

getDescriptor - ifdefed
AParcelableHolder - simplified - this was added in API level 31
  so lifting checks to be about the whole class

Bug: 257135519
Bug: 219987524
Test: build w/ `-U__ANDROID_UNAVAILABLE_SYMBOLS_ARE_WEAK__`
Test: atest CtsNdkBinderTestCases
Test: atest CtsNdkBinderTestCases (w/ -U__ANDROID_UNAVAILABLE_SYMBOLS_ARE_WEAK__)
Change-Id: I8e0b03b775609b1c987da42a54ed070a64eda201