Binder interface tokens: remove extra mallocs

- writeInterfaceToken/enforceInterfaceToken try to get the interface
  token with GetStringRegion, so for classes with short enough
  interface descriptors, the string is allocated on the stack. A size
  of 64 was chosen since the longest interface descriptor in a
  frameworks/base AIDL file is ~40 characters.
- remove extra creation of String16 for writeInterfaceToken

Bug: 169674485
Test: atest aidl_integration_test
Change-Id: I78092abaa4df758ab699fcb103aa619b0b426e6c