Add proguard rules.

[tiper]

PdfBox-Android does reflection to instantiate SecurityHandlers.

[TomRoush]

This looks pretty good, thank you for contributing. I did have a few questions:

• Can the rules file be moved to library/consumer-proguard-rules.txt? The Proguard file doesn't need to go in META-INF for an AAR library
• We will also need to add consumerProguardFiles 'consumer-proguard-rules.txt' to library/build.gradle
• What issue were you having before this change was made?

[tiper]

Hi @TomRoush.

• Can the rules file be moved to library/consumer-proguard-rules.txt? The Proguard file doesn't need to go in META-INF for an AAR library.

✅

• We will also need to add consumerProguardFiles 'consumer-proguard-rules.txt' to library/build.gradle

✅✅

• What issue were you having before this change was made?

Here's a stack trace when replicating the issue. Should be better than any explanation 😛

java.io.IOException: Error (RuntimeException) while creating security handler for decryption
    at com.tom_roush.pdfbox.pdfparser.COSParser.void prepareDecryption()(Unknown Source:2922)
    at com.tom_roush.pdfbox.pdfparser.COSParser.com.tom_roush.pdfbox.cos.COSDictionary retrieveTrailer()(Unknown Source:285)
    at com.tom_roush.pdfbox.pdfparser.PDFParser.void initialParse()(Unknown Source:169)
    at com.tom_roush.pdfbox.pdfparser.PDFParser.void parse()(Unknown Source:222)
    at com.tom_roush.pdfbox.pdmodel.PDDocument.com.tom_roush.pdfbox.pdmodel.PDDocument load(com.tom_roush.pdfbox.io.RandomAccessBufferedFileInputStream,java.lang.String,java.io.InputStream,java.lang.String,com.tom_roush.pdfbox.io.MemoryUsageSetting)(Unknown Source:1086)
    at com.tom_roush.pdfbox.pdmodel.PDDocument.com.tom_roush.pdfbox.pdmodel.PDDocument load(java.io.File,java.lang.String,java.io.InputStream,java.lang.String,com.tom_roush.pdfbox.io.MemoryUsageSetting)(Unknown Source:1069)
    at com.tom_roush.pdfbox.pdmodel.PDDocument.com.tom_roush.pdfbox.pdmodel.PDDocument load(java.io.File,com.tom_roush.pdfbox.io.MemoryUsageSetting)(Unknown Source:993)
    ...
 Caused by: java.lang.RuntimeException: java.lang.NoSuchMethodException: com.tom_roush.pdfbox.pdmodel.encryption.StandardSecurityHandler.<init> []
    at com.tom_roush.pdfbox.pdmodel.encryption.SecurityHandlerFactory.com.tom_roush.pdfbox.pdmodel.encryption.SecurityHandler newSecurityHandler(java.lang.Class,java.lang.Class[],java.lang.Object[])(Unknown Source:137)
    at com.tom_roush.pdfbox.pdmodel.encryption.SecurityHandlerFactory.com.tom_roush.pdfbox.pdmodel.encryption.SecurityHandler newSecurityHandlerForFilter(java.lang.String)(Unknown Source:115)
    at com.tom_roush.pdfbox.pdmodel.encryption.PDEncryption.void <init>(com.tom_roush.pdfbox.cos.COSDictionary)(Unknown Source:97)
    at com.tom_roush.pdfbox.pdfparser.COSParser.void prepareDecryption()(Unknown Source:2898)
    ...
 Caused by: java.lang.NoSuchMethodException: com.tom_roush.pdfbox.pdmodel.encryption.StandardSecurityHandler.<init> []
    at java.lang.Class.getConstructor0(Class.java:2363)
    at java.lang.Class.getDeclaredConstructor(Class.java:2201)
    at com.tom_roush.pdfbox.pdmodel.encryption.SecurityHandlerFactory.com.tom_roush.pdfbox.pdmodel.encryption.SecurityHandler newSecurityHandler(java.lang.Class,java.lang.Class[],java.lang.Object[])(Unknown Source:131)
    ...

I've updated the PR. Let me know if there's anything else that needs to be done.