Fix clang static anaylzer warnings.

Initialize the values in readBool, readChar and readByte functions.
Silences the following warnings:
frameworks/native/libs/binder/Parcel.cpp:1856:18: warning: The left
operand of '!=' is a garbage value.
frameworks/native/libs/binder/Parcel.cpp:1869:11: warning: Assigned
value is garbage or undefined.
frameworks/native/libs/binder/Parcel.cpp:1882:11: warning: Assigned
value is garbage or undefined.

Bug: b/27101951
Test:Warnings are gone.
Change-Id: Iba8d242d4d9b1b48f7cafb901023133e21d9d732